1. Field of the Invention
The present invention relates to a zoom microphone device, and more particularly, to a zoom microphone device having an audio zooming function which allows a target sound to be picked up with an effective enhancement in accordance with a zoom position.
2. Description of the Background Art
In the field of video cameras and digital cameras having the ability of imaging moving pictures, etc., zoom microphone devices are conventionally available which are capable of zooming in on a target sound in synchronization with a zooming motion of a lens so as to pick up the target sound with a high SNR (signal-to-noise ratio). Examples of methods for realizing such a zoomed picking-up of sounds include methods which involve simple frequency compensation, and methods which involve altering the directivity characteristics of a microphone through digital signal processing. Hereinafter, conventional zoom microphone devices utilizing these methods will be briefly described with reference to the accompanying drawings.
As a first conventional example, FIG. 21 illustrates a zoom microphone device structure which realizes zoomed picking-up of sounds with a simple frequency compensation technique. The zoom microphone device of the first conventional example includes a pickup section 900, a zoom control section 901, and a high-pass filter 902. The pickup section 900 transduces sounds to an audio signal. The zoom control section 901 outputs a zoom position signal which determines a zoom position. The high-pass filter 902 enhances a high-frequency range of the audio signal that is outputted from the pickup section 900, the frequency characteristics thereof being adjusted in accordance with the zoom position signal which is outputted from the zoom control section 901. This adjustment occurs in such a manner that the high-frequency range of an input audio signal is more enhanced as the zoom position is moved closer to the telescopic end from a wide-angle end.
Sounds which are input to the pickup section 900 usually include target sounds as well as some background noise. Under a telescopic operation, target sounds are typically generated at a relatively remote location from the zoom microphone device. The ambient noise generally has a spectrum which is relatively concentrated in the low-frequency ranges. Therefore, under the telescopic operation, the low-frequency ranges of the audio signal which is output from the pickup section 900 may be cut off by means of the high-pass filter 902 so as to relatively reduce the proportion of the background noise in the audio signal. Thus, an improved SNR can be provided under the telescopic operation which enables zooming effects.
As a second conventional example, FIG. 22 illustrates a zoom microphone device structure which realizes zoomed picking-up of sounds by altering the directivity characteristics of a
microphone through digital signal processing. The zoom microphone device of the second conventional example includes a pickup section 903, a zoom control section 904, a directivity control section 905, and a volume control section 906. The pickup section 903 includes microphone units 907a and 907b. The directivity control section 905 includes: an adder 908; amplifiers 909, 910a, 910b and 910c; and adders 911a and 911b. 
The microphone units 907a and 907b are oriented at certain angles with respect to a frontal direction of the zoom microphone device. The adder 908 adds the respective audio signals that are outputted from the microphone units 907a and 907b. The amplifier 909 multiplies the amplitude of the resultant added audio signal by 0.5. The amplifiers 910a, 910b, and 910c adjust the amplitude levels of the audio signals that are outputted from the microphone units 907a and 907b and the amplifier 909, respectively, in accordance with a zoom position signal which is output from the zoom control section 904. Specifically, under a wide-angle operation, the gain of each of the amplifiers 910a and 910b is set to “1”, and the gain of the amplifier 910c is set to “0”. On the other hand, under the telescopic operation, the gain of each of the amplifiers 910a and 910b is set to “0”, and the gain of the amplifier 910c is set to “1”. The adder 911a adds the output from the amplifier 910c to the output from the amplifier 910a, thereby outputting an R channel audio signal. The adder 911b adds the output from the amplifier 910c to the output from the amplifier 910b, thereby outputting an L channel audio signal.
Sounds which are input to the pickup section 903 usually include target sounds as well as some background noise. Under the telescopic operation, target sounds are typically generated in the frontal direction of the zoom microphone device, while the background noise occurs in an omnidirectional manner. Therefore, under the telescopic operation, the directivity of the R channel and the L channel may be oriented toward the frontal direction so as to reduce the proportion of the background noise in the audio signals of the respective channels in a relative manner. Thus, an improved SNR can be provided under the telescopic operation which enables zooming effects.
The zoom microphone device of the second conventional example includes the volume control section 906 for the following reason. In general, the source of a target sound under the telescopic operation is located farther away from the source of a target sound under the wide-angle operation. Therefore, a target sound under the telescopic operation has a relatively low sound volume when picked up by the zoom microphone device. Accordingly, the volume control section 906 is used to increase the sound volume of the audio signals of the respective channels under the telescopic operation, whereby zooming effects can be obtained.
However, according to the first conventional example as illustrated in FIG. 21, not only the low-frequency range of the ambient noise but also the low-frequency range of the target sound is cut off by the high-pass filter 902 under telescopic operation. Therefore, the tone (i.e., frequency characteristics) of the target sound may vary as the zoom position is changed.
According to the second conventional example as illustrated in FIG. 22, there is a problem in that any sound (i.e., not only the target sound but also the constantly-standing background noise) that comes from the frontal direction of the zoom microphone device under telescopic operation will be picked up, and as a result, the SNR may not be sufficiently improved.
There is also a problem with the technique of increasing the sound volume level under the telescopic operation through volume control in that not only the target sound but also the background noise level is inevitably increased. Therefore, this does not improve the SNR so as to sufficiently enhance the target sound.